576 



Dr. W. B. Carpenter on the 



[June 13, 



and the buoy moved in the same direction at the rate of 1*35 mile per hour, 

 the real rate of the under-current was estimated at 1*35 + 0*1 7, or 1-52 mile 

 per hour. On the other hand when the surface-current was running at 4 '4 

 miles per hour, and the buoy moved in the same direction at the rate of 

 3*2 miles per hour, the real rate of the under-current was estimated at 

 3*2 — 0'7, or 2*5 miles per hour. 



61. In the event of any renewal of such experiments, either in the 

 Strait of Gibraltar or elsewhere, I should recommend that the relative re- 

 sistances of the Current-drag and of the Suspending-buoy at different rates 

 of movement should be tested, in the first instance, by drawing them sepa- 

 rately through still water with a Dynamometer attached to the line ; and 

 further, that by suspending the current-drag at different depths, and by 

 testing the resistance then offered in each case by the drag and the line 

 conjointly, the amount due to the latter should be determined by the eli- 

 mination of the former. If in this manner the amount of resistance that is 

 offered by (say) every 20 fathoms of the line from which the drag is 

 suspended be ascertained, we shall have satisfactory data for determining, 

 with a near approach to accuracy, the actual rate of the current in which 

 the drag is suspended. For as it may be pretty certainly determined by 

 Specific Gravity observations to what depth the upper stratum extends at the 

 point of observation, it will be easy to compute, on the one hand, the force 

 exerted by its current upon the suspending-buoy and on the upper part of 

 the line that hangs from it ; and, on the other, the force of the under- 

 current, not only upon the drag, but upon the portion of the suspending 

 line that hangs in it : and from these data the actual rate of the under- 

 current can be readily worked out with a near approach to accuracy. 



62. Temperature of the Strait of Gibraltar. — It will be remembered 

 that in the previous year's work a marked reduction of Temperature was 

 observed in the mid-stream of the Strait, as compared with the water 

 nearer the Spanish side ; and the inference dra wn from these observations 

 was, that " either the water of which the in-current consists is drawn from 

 " a part of the Atlantic at least as far north as Lisbon, or that it is derived 

 " from a stratum of the neighbouring ocean somewhat beneath the surface, 

 " so as to have received less of the solar superheating than the actual surface- 

 -water" (Report for 1870, § 74). On the other hand, the excess of 

 temperature in the surface-water of the northern side of the Western em- 

 bouchure of the Strait was considered as indicating that there is a predo- 

 minant surface-outflow of Mediterranean water along the Spanish coast ; 

 a fact, I was informed, well known to those who have navigated it (§ 75). 

 — Being desirous of obtaining further information on this point, I requested 

 Capt. Nares to take observations of surface-temperature at short intervals, 

 on the two occasions on which we were running obliquely across the 

 Strait. One of these series (I.) extends along a line of about 10 miles from 

 the neighbourhood of the Pearl Rock, at the entrance of Gibraltar Bay, to 

 a short distance west of Point Cires on the African Coast ; the other 



